The minimum angle for the "tube" to work is like 20 or 25deg.
I can see a lot of easy ways to have this deployed on a boat...imagine an elongated bladder or even a rigid manifold that these tubes would plug into....supply side a hose from any cold water faucet...discharge a hose (insulated is better) that goes to a deck fitting that is plumbed to a "y" valve on the supply line of the water heater.
If you really want a lot of tubes maybe your boat has a built in "ring" manifold all the way around the gunnels...and you plug the tubes is as required....they wouldn't make very good fenders.
I think Fish may have the right idea....purpose build the push and pull pits or arch or boom for that matter to be manifolds....lots of ways to go.....depending on how much benefit we'll get from that heat....and it seems there are lots of possibilities there.

It's hard to distinguish the humor from the science in this one. But I'll add a few facts and opinions on the subject (as a chemist). First, the total weight (mass) of hydrogen and oxygen needed to make water is exactly the same as the weight of water that you receive. Second, there is no hydrogen roaming around in the air. In fact, hydrogen is so light that it has "escape velocity", i.e., if you release hydrogen it will eventually escape the earth's magnetic field and drift off into space.

I never understood the idea of a hydrogen economy! It is one of the worst things to handle. It isn't explosive by itself but it forms explosive mixtures in air at a really wide range of concentrations. It is damned near impossible to contain. It leaks out of most materials and "reacts" with most metals to make them brittle and porous. Systems that use hydrogen are always replacing the tubes, valves, etc. I think I'd rather have an equal weight of military high explosive on my boat than hydrogen.

I think most commercial hydrogen comes from oil refining. But the fuel companies use it as fast as they make it because it improves the quality of their fuels (when reacted with the feedstock). You can make hydrogen from seawater. But as pointed out: It takes a LOT of energy. We don't "make" energy except in nuclear reactors, including the sun!. We just collect it, store it and move it around. It turns out that dieselfuel is just about the highest energy density that you can get. If I'm not mistaken diesel has about three or four times the energy per gram as TNT. It's just that TNT releases it faster! You can certainly collect energy from the sun with efficiencies around 20% for the conversion to electricity.

Most scientists learn something about Thermodynamics. There are lots of thermodynamic laws and principles (including Boyles Law) but there are three fundamental laws of Thermo. In layman's terms they are:
1. You can't get something for nothing. (Conservation of Energy)
2. You can only break even at absolute zero. (Entropy)
3. You can't reach absolute zero.
So, you are always going to lose energy. And that's OK for most things but don't look for perpetual motion!

Enough hydrogen-oxygen-dehydrated water stuff.

But, water is a big problem and there is a lot of research in trying to come up with better ways to get it...mostly get the salt out of it. So far RO seems to be the best but it does take energy. It's actually the second law above that comes to play here. And there is a lot of effort going into better membranes. But one of the biggest problems is fouling. The more efficient membranes tend to foul faster. Keep our fingers crossed, better ones are on the way. I think that carbon nanotubes are probably useful for making a strong, thin membrane because they have unbelievable strength properties. They are probably not (at this time anyway) useful as pores because most nanotubes are not open from end to end.

As for fuel cells, they don't generate enough water to keep a flea alive. Most of them don't generate enough to keep their membranes wet. And hydrogen fuel cells have all of the disadvantages of managing hydrogen gas. But there is a lot of work on hydrocarbon and alcohol fuel cells that would really help the cruiser run all of our "needed" systems. The holy grail here is a fuel cell that will "burn" JP-8 (diesel). But it's the catalyst/membrane that's holding that up as well. Maybe one of these days.

As a chemist, what do you think of concentrating sunlight via a reflector to boil sea water to recover fresh water via distillation?

I once burned a 1" hole in a plank of wood with sunlight focused using a 10" astronomical mirror. You can concentrate a lot of heat with a small mirror. When the summer hits I might try making a 1 metre parabolic reflector and see how it goes.

I'll try to give some general answers to those two questions: 85 C heat and using reflectors. First, understand that in most thermodynamic processes or systems it's degrees Kelvin that count and the systems become more efficient and produce more energy if you can change the temperature over a lot of degrees. For most of us the source of "cold" will be the water around the boat. That probably ranges from 45 to 80 Fahrenheit so pick 65. That's about 18 C or 281 K. And 85 C is 358 K, that's not a really big difference. The other thing is that water has a very large heat of vaporization, the amount of heat required to take it from liquid to gaseous state. In simplest terms it takes one calorie to raise a gram of water one degree C or K. But it takes 539 calories to take one gram of water from liquid to gas. So, you'd have to take 8 grams of water from 85 to 18 C in order to vaporize one gram of water. That's with perfect efficiency. So, if you've got a lot of water at 85 C (obviously we've got a lot at 18 C) then you can do it. The problem is that it will probably take a pretty large and complex "machine" to do it.

As for using reflectors...Yep! I'd say that most solar thermal technology is pretty well developed. There are excellent reflector based heaters available. So you can surely concentrate the sun's heat and boil water. But the problem is going to be condensing it. Clearly the oil and chemical industry have developed good condensers but they tend to be large to get any efficiency. And you are going to have to pump all of that seawater around to cool it. Those are referred to as parasitic losses. One could clearly design a system to do it. But again, I'd be afraid that it would be large and unwieldy on a sailboat. And as I am reminded everytime I look around my boat: salt water corrodes the hell out of everything. So there would be some materials issues. All could be resolved but I wonder about the price and lifetime of the equipment.

Now here's an idea that might work, see if anyone wants to fund it!! You could use the temperature difference between a solar hot plate (just some copper that is painted black for the simplest one) and the ocean water to drive a Sterling engine. They tend to be pretty efficient. The Stirling engine could power a pump which would then pump the salt water through an RO membrane. If you want to have some fun, Google Stirling engines and see some of the clever ones that folks have made. Lots of Youtube and other videos out there.

Human kidneys mimic RO membranes and also need flushing to clear the tissue of salts. Its one reason why we dehydrate when we drink coffee and alcohol

So how about a mail sail with 3 layers. pump small amounts of salt water up to the head and allow to perculate down between two of the layers. the inside middle layer mimics the human kidney allowing water through into the gap between the middle and the other side. Need a bit of heat, then make it a black sailcloth. Doesn't need to be overly efficient as you have a huge surfice area available and its easily stowed.

I'm thinking some of us don't spend enough time sailing to have time to read :-)
I do remember seeing something many years ago ...I think it was in the Andies...it was a long mesh, strung up like a fence with a gutter under it...it collected the moisture from the clouds on the mesh then it dripped down to the gutter.

Err... I have read it (several times) - I've even laughed at the movie, but it is fiction. Having said that reality has a tendency to catch up with fiction after a while. Anyone remember the 1936 "The Shape of Things to Come"? People routinely travelling by aeroplane and politicians denouncing each other on giant video screens? Maybe "Dune" technology will arrive. Maybe it already has (see below)

"....in the Andies...it was a long mesh, strung up like a fence with a gutter under it...it collected the moisture from the clouds on the mesh then it dripped down to the gutter."

I saw a variation on this on a science programme on the BBC 10 or 15 years ago where they mounted a little building on a slope in the Canaries. Basically, it took sea water and run it down some mesh. The mesh faced into the sea breeze and the breeze collected some water vapour. This blew through the "building" (more of a shed really) to a cold metal surface on the back where it condensed and ran into a gutter. It produced a couple of hundred litres of fresh water every day and the small pump was run by a solar panel.

Having said that reality has a tendency to catch up with fiction after a while.

Indeed... remember the communicators that Captain Kirk and his crew used.... look at our cellphones now!

But in the case of wind-traps, I think the technology has been around for a long time. The fiction part in Dune was that they could generate water while there was basically no humidity in the air. We all see condensation on our boats, it comes from the air, we try to prevent it, we could build windtraps and catch it! We could also "take the water" from the creepy types we meet during our travels! (joke only Dune freaks understand ;-)

Edit: p.s. if you could bring yourself to see that lousy movie start to end, you really should get that Australian mini-series too because it's much better than that movie!!